Method of treating chromic oxide



July 26, 1955 S. E. VOL-rz 2,714,054

METHOD OF' TREATING CHROMIC OXIDE Filed Feb. l5, 1952 CHROMIC OXIDE lll/lll.

T TORNEV United States Patent ce Patented `i'uly 2, IQS

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Sterline E. Volta Norwood, Pa., assigner to Hendry a a A. Process Corporation, Wri nngton, Del., a corporation of Delaware Application February l5, 3%2, Serial. No. 271,762 3 Claims. (Cl. 23m-idd) This invention relates to temperature responsive devices, and more particularly to a temperature responsive resistance employing a specially treated chromic oxide having a negative temperature coefficient.

It is well-known in the art relating to resistances that certain types of resistance materials, referred to as semiconductors and comprising in general metal oxides, have a negative temperature coefficient. in the case of a resistance material having a positive temperature coeiicient, if such a resistance is placed in an electrical circuit, the current in the circuit will be reduced as the temperature rises due to the rise in resistance. Temperature responsive devices comprising materials of the aforementioned type may be used in association with various control devices or may be employed merely to record temperature. Generally speaking, those resistance inaterials having a negative temperature coefficient also have a larger temperature coefficient.

It is an object of this invention to provide a temperature responsive device of the foregoing type having a negative coefficient of resistance.

Another object is the pro'v n of a temperature rcsponsive device formed from cnromic oxide.

A further object is to provide a temperature responsive device of the foregoing type whose range of resistance is reliably stable throughout extended temperature ranges.

In order to more completely describe ity invention, reference will be made to the accompanying drawing wherein Figure 1 shows a cross-sectional view of a temperature responsive element;

Figure 2 illustrates a moditicd temperature responsive element; and

Figure 3 shows one type of circuit in which a temperature responsive device may be employed.

In accordance with the teaching of my invention, l obtain a suitable mass of chrom-ic oxide and subject it to a treatment as will be set forth hereinafter. One source of chromic oxide which l have found satisfactory is obtained by precipitation from dilute solutions of chromic nitrate and ammonium hydroxide. Following precipitation, the precipitate is washed and then dried at 110 for eight (8) hours. it was found, on analysis. that the gel contained 67.82% of CrzCs. after this initial drying, the sample was further treated by contacting the same with a stream of nitrogen at 350 C., following which an analysis was made disclosing the material to contain 91.24% to 92.48% of C1203, the increase being due primarily to a further loss of water. lt has been found that, in addition to water, the chromic oxide may contain as much as 5 .4% foreign material, including CG-z, N2 and NO, which may also be removed during either the nitrogen treatment or the subsequent oxygen treatment described hereinafter. The removal of these addi tional impurities is important in that their presence in the finished temperature responsive resistor may adversely affect the stability of the resistors when subjected to a relatively large temperature range.

A mass of chromic oxide treated in the foregoing manner is next subjected to a treatment with oxygen for several hours to insure establishment of the stable form of chromic oxide. Thus, a treatment of approximately 20 grams of the chromic oxide for two (2) hours with oxygen at 350 C. and at the rate of approximately 5 liters per hour gave a black product that was 99.17% CrzOs.

This product was subjected to resistance measurements wherein the resistance was found to be low at high temperatures and high at low temperatures, hence having a negative temperature coetlicient. For example, a sample of this type of chromic oxide having a bulk density of about 1.3 benzene of 4.7-4.99) and weighing 12.86 grams was tested in apparatus of the type shown in Figure 1. The apparatus comprised a brass outer shell, 3 inches in length with an internal diameter of 11/16 inch, which served as one electrode; the other electrode was a brass rod, 3/8 inch in diameter, which was placed coaxially within the shell. The annular space between the shell and the rod was occupied by the chromie oxide suitably heid in place by nonconducting washers or rings which likewise served to determine and maintain the relative positions of the shell and rod. Resistance of this chromic oxide resistor at 350 C. was of the order of l03 ohms; and at -l C. the resistance was of the order of 108 ohms. Thus, for a change of 545 C. there was a corresponding resistance change of ohms. The change in resistance was substantially linear over this range according to the formula in which the logarithm of the resistance is proportional to the reciprocal of the temperature in degrees absolute.

While the foregoing description has shown temperatures of 350 C. for stabilizing and use of the chromic oxide, this temperature is not to be construed as the upper limit inasmuch as higher temperatures have been satisfactorily employed, as up to 500 C. However, in order to insure the stability of the chromic oxide the temperature of the stabilizing treatment should not be exceeded during subsequent use. Thus, it is preferable to employ the stabilized chromic oxide as a temperature responsive means at temperatures no higher than about 25 C. below the temperature of the preceding stabilizing treatment.

While the foregoing material may be used Without further treatment, it is within the scope of this invention to treat the chromic oxide so formed at 350 C. with hydrogen. Such a treatment, however, substantially increases the resistance characteristics of the chromic oxide and likewise reduces the temperature coeticient (or resistance change caused by temperature change). The chromic oxide materials treated in this manner, while controllably adjustable through a relatively wide range of resistance values as more fully set forth in my copending application hereinafter identified, have relatively limited uses as temperature responsive, e. g., resistance thermometer, elements because of their curtailed temperature coefficient. However, it is additionally within the scope of this invention to subject such hydrogen treated materials to further treatment with oxygen, also at elevated temperature in the chemisorption range, in the manner outlined in my companion patent application Serial No. 271,761 and thereby adjust the basic resistance and temperature coefficient to any desired intermediate value.

Resistance elements of the type disclosed in this invention have been evacuated and sealed, while other samples have been evacuated and sealed in a suitable inert atmosphere such as argon or helium. in each case, satisfactory resistance measurements have been obtained. Indeed, it is preferable to have these temperature responsive resistance elements, which are sealed from the surrounding atmosphere to prevent undesired changes in their characteristics caused thereby, to be sealed in a positive gas pressure in that their response to temperature change will be appreciably more rapid than when sealed in vacuo.

In experiments conducted with the chromic oxide prepared in accordance with this invention, resistance measurements obtained from both relatively loosely packed chromic oxide powder and powder which had been subjected to some pressure Were comparable. In forming the powder into any desired shapes, only binders should be employed which may be subsequently removed, rather than of the permanent type, such as, for example, some of the high temperature stable resins.

It is to be understood that other methods of obtaining the chromic oxide are within the scope of my invention. Likewise that other conditions, including ternperature, may be employed. In the interests of stability in subsequent use it is desirable to have the chromic oxide in high purity such as 97% or better and preferably better than 99%.

Referring now to the drawing illustrating various applications of this invention, there is shown in Figure l a temperature responsive device containing the chromic oxide prepared in accordance with the present invention. The numeral 1 designates a tubing having disposed coaxially therewith a conducting rod 2. In the annulus between the tubing and rod, there is shown a mass of chromic oxide 3. Two porcelain washers 4 are employed at the top and bottom of the tubing to retain the chromic oxide in place and to hold in spaced relationship therewith brass rod 2. Suitable leads 5 and 6 are connected to the electrodes tube 1 and rod 2, respectively. The entire apparatus of Figure l may be enclosed in a glass tubing which, in turn, may be sealed under vacuum or suitable inert gas pressure.

In connection with the structure disclosed by Figure 2, there is shown a modified temperature responsive device wherein the numeral 8 designates a glass envelope containing the chromic oxide 9 of the present invention. Adjacent the ends of the chromic oxide mass 9 are two electrodes comprising metal disks 10 and 11. Connected to the electrodes and extending outwardly from the glass envelope 8 are leads 12 and 13 adapted to be attached to any suitable electrical connections to form part of a circuit in the manner illustrated in Figure 3. In this latter gure illustrating one use of my invention, there is shown an electrical circuit indicated generally by the numeral 14 having, in series, a temperature responsive device 15, a source of voltage 16 and an indicating means 17. Thus,

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in use, the device 1S is subjected to heat from an appropriate source; and, on a rise in temperature, its resistance decreases allowing an increase in current, thereby causing a change in the reading of indicating means 17. Means 17 may be calibrated so as to indicate temperature directly.

I In a similar manner, the chromic oxide of this invention may be employed in a variety of thermo-electrical systems, such as' relay and Wheatstone bridge type circuits.

While certain specific embodiments of the present invention have been shown and described, it will be readily understood by those skilled in the art that the same is not to be limited exactly thereto, since various modifications may be made without departing from the scope of the invention as deiined in the appended claims.

I claim:

l. A method of making a temperature responsive resistance element having a negative temperature coefficient comprising treating a mass of chromic oxide with oxygen at 350 C. until substantially all of said chromic oxide has the atomic ratio of two atoms of chromium to three atoms of oxygen, and forming said mass into a predetermined shape.

2. A method of making a temperature responsive resistance element having a negative temperature coeicient comprising treating a mass of chromic oxide with a neutral gas at 350 C. to remove moisture and adsorbed gases, evacuating the atmosphere surrounding said mass, and then treating said mass with oxygen at 350 C.

3. A method of making a temperature responsive resistance element for use in association with resistance responsive means, which comprises stabilizing chromic oxide in the purity of at least 99% by treatment in an oxygen-containing atmosphere at a temperature in the range of 350 to 500 C., further treating said chromic oxide in a hydrogen containing atmosphere at elevated temperature, and subsequently adjusting the basic resistance and temperature coefiicient of said chromic oxide by additional treatment with oxygen at elevated temperature in the chemisorption range.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF MAKING A TEMPERATURE RESPONSIVE RESISTANCE ELEMENT HAVING A NEGATIVE TEMPERATURE COEFFICIENT COMPRISING TREATING A MASS OF CHROMIC OXIDE WITH OXYGEN AT 350*C. UNTIL SUBSTANTIALLY ALL OF SAID CHROMIC OXIDE HAS THE ATOMIC RATIO OF TWO ATOMS OF 